1. Field of the Invention
The invention relates to a method of making rings for gears having internal or external toothing and the use of such an integral ring for making internally or externally toothed gears.
Robots, in particular industrial robots, must be very stiffly constructed in order to achieve good positioning accuracy. Since as a rule conventional materials such as steel or cast iron are used, large and poor acceleration values result, due to the large masses of the drives and in particular of the transmissions. Drives and transmissions which are frequently used in such cases are those of the company Harmonic Drive, which hereinafter will be referred to as Harmonic Drive transmissions.
Harmonic Drive transmissions of the HDUC series consist essentially of three components or assemblies, that is a so-called wave generator (WG), a so-called flexible spline (FS) and a circular spline (CS).
The wave generator here has the form of an elliptical disc with central hub and an externally arranged special ball bearing. The flexible spline is a cylindrical steel bush with external toothing and has a flange at the bottom. The circular spline is a ring having internal toothing. All three components are made from iron alloys.
The elliptical wave generator which acts as the driven part in this case, deforms the externally toothed flexible spline via its ball bearing. In the oppositely disposed regions of the major ellipse axis, about 20% of all the teeth are in continuous engagement with the internally toothed circular spline, which has two teeth more than the flexible spline. By rotation of the wave generator the tooth meshing with the internally toothed circular spline is displaced corresponding to the major ellipse axis. As a result, after half a revolution of the wave generator a relative movement through one tooth takes place between the flexible spline and the circular spline, preferably mounted on the housing, or through two teeth after one complete revolution.
If, for example, the circular spline is fixed, the flexible spline rotates oppositely to the wave generator. The input-torque is therefore imposed on, the wave generator, whereas the output-torque can be taken off the flexible spline. To balance the difference of input-torque and output-torque the circular spine therefore must be mounted on the housing of the transmission. The wave generator therefore serves as drive-side component whilst the circular spline is employed for mounting and supporting on the housing and the flexible spline is the driven component.
A mass division for two Harmonic Drive transmissions is shown in the following Table I:
TABLE I ______________________________________ HDUC 80 IH HDUC 32 Proportion Proportion of the total of the total Type Weight weight Weight weight ______________________________________ Circular 7.60 kg 48.1% 0,60 kg 50.0% Spline Flexible 2.45 kg 15.5% 0.25 kg 20,8% Spline Wave 5.75 kg 36.4% 0.35 kg 29.2% Generator Total 15.8 kg 100% 1.20 kg 100% Weight ______________________________________
As apparent from Table 1, with a mass proportion of about 50% the circular spline makes the greatest contribution to the total mass.
In the tooth meshing region of the circular spline, the peripheral load and the tooth geometry result in a radially outwardly acting force component which effects a widening of the metal ring. The circular spline must therefore have a very high deformation stiffness so that a tooth jumping cannot occur between the flexible spline and the circular spline due to high torques or short overloading of the transmission. Consequently, this above described load does not result in a pure bending stresses in the material but includes particularly pronounced shearing loads. So far the hitherto used isotropic material metal is fundamentally very well suited to this type of load.
For introducing the forces or moments a flange mounting is employed, i.e. in the external ring of the circular spline mounting bores are arranged along a circular line. Here, the hitherto used material metal guarantees an adequately high hole bearing strength of these bores. In addition, in the construction of the entire gear from metal a substantially uniform coefficient of thermal expansion is ensured and consequently at elevated temperatures there is no danger of seizure.
A disadvantage of the conventional Harmonic Drive transmissions is the excessive mass. If such a transmission is employed in an articulate drive of a robot, in particular in robots having long arms this excessive mass leads to high torques in the drives lying closer to the base. To achieve an adequate positioning accuracy the structure and drives must therefore be made very stiff and this again results in large masses. This in turn leads to the mass which can be transported being small compared with the robot total mass and also enables only small accelerations to be achieved.